Capitalization would be correct in almost all cases. You always capitalize first letter and chances are, first word of each line won't be the beginning of a sentence. Ditto for spacing, 3-5 line paragraphs are fairly common.
How do you figure "almost all"? Even if the text could only contain paragraphs of length 3 and 4, the spacing alone would be incorrect in half the cases (assuming that a 3-4 and 4-3 split is equally likely).
(Of course, we are leaving aside the fact that the full body of the message contains an additional one-line paragraph, which could also be considered part of a correctly capitalized and spaced message.)